The reason I said maybe it's under the winter coat is because I have a flea bitten that has dapples in the summer. He is snow white in the winter. I've seen many horses that are much different coloring in a winter coat then a summer coat. And when I mentioned the tiger striping and the dorsal stripe you'll notice I said "I'd have thought dun".
I haven't really seen any "colored" brindles that the brindle hides under the winter coat. If that makes sense. I mean it is entirely possible, but I truly don't think that the horse on the OP is anything more than dun.